Disk drives employing a spindle assembly for rotatably mounting rigid data storage disks relative to a base or frame of the disk drive are known in the art. A disk spindle assembly includes a vertical spindle hub which is journalled by bearings to the base or frame so as to be rotatable relatively thereto. Typically, a flange extends radially outwardly from the hub at a bottom region thereof. A rigid storage disk of the type presently under contemplation is typically formed of sheet metal such as aluminum alloy which is provided with a magnetic storage media formed on the two major surfaces thereof. A central opening enables the disk to fit coaxially over the spindle hub. A single data storage disk may be used in the disk drive; or, multiple data storage disks may be provided in a stacked, spaced apart relationship. Rigid annular spacers are used to space the disks apart. A top clamp is typically provided to clamp the disk stack including the disks and spacers downwardly upon the outwardly extending radial flange of the hub.
The function of the spindle assembly is to provide two interfaces with the storage disk(s): an axial interface for fixing the disk in space vertically along the longitudinal axis of rotation of the hub, and a radial interface for fixing the disk concentrically with the axis of rotation. To provide effectively these two interfaces the spindle assembly must clamp the disk in axial position and must clamp the disk in radial position with sufficient clamping force that the disk remains properly positioned throughout the useful life of the disk drive.
The spindle assembly is rotated by a motor so that a relatively stationary, yet positionable head may "fly" upon an air bearing formed at the data storage surface of each data storage disk disk in order to carry out data transfer operations relative to multiple concentric data tracks formed thereon. The spindle motor may be external to the spindle assembly and transfer rotational energy by a belt or other driving arrangement. The motor may be directly coupled to the spindle, yet external to it. More frequently today, the motor may be formed entirely within the spindle; and when it is, certain problems relating the the mounting of the disk stack upon the spindle assembly arise.
Since an in-the-hub spindle motor generates heat as it generates and imparts rotational force to the disk spindle, the heat causes the spindle assembly to expand according to the coefficient of expansion of the various materials from which the spindle assembly is formed. The hub tends to expand radially outwardly with heat, and this outward expansion may cause the disks to slide within the clamped stack (radial displacement), or to become warped (axial displacement). Disk slippage leads to eccentricity of data storage tracks and potential loss of data. Warpage leads to disks being out of flatness and potential catastrophic interference between the flying head and the disk surface. Within a disk stack of four disks, for example, the upper and lower disks may experience both axial and radial displacements, whereas the middle disks and spacers between the disks tend to experience radial displacement only.
Also, an in-the-hub spindle motor typically requires that a magnetic return path be provided. Such a path is typically provided by a magnetic flux carrying material, such as low carbon steel. The coefficient of thermal expansion of an aluminum alloy storage disk is typically approximately 13 microinches per degree Fahrenheit. The coefficient of thermal expansion of low carbon steel is approximately 6.5 microinches per degree Fahrenheit. A hub assembly formed as a composite of an aluminum alloy casting and a low carbon steel flux return insert has an overall coefficient of thermal expansion of about 9 microinches per degree Fahrenheit.
During operation of the disk drive, changes in temperature within the drive cause expansion and contraction of the disks and of the spindle assembly including the hub. The differences in thermal expansion coefficients will result in more expansion and contraction of the aluminum alloy disk than of a composite aluminum/steel hub assembly. As already noted, the position of the disk relative to center of rotation of the hub may shift on account of thermal expansion and contraction. Over time, such shift results in eccentricity of the data tracks of the particular disk relative to the spindle center of rotation. In practical terms, such a shift may lead to lost data: i.e., data that was recorded when the disk was concentric that now cannot be recovered because of the shift which has resulted in runout or eccentricity.
One prior approach to compensate for disk slippage has been to subject newly assembled disk drives to thermal cycling at the end of the manufacturing/testing process. While this approach may tend to stabilize disk drives, it does not eliminate slippage, and it is time consuming and adds to the cost of manufacture of the completed disk drive.
One other approach has been to increase the clamping force by which the disk stack is clamped onto the flange of the spindle hub. Very large clamping forces have been used in an attempt to maintain the disks in proper position over extended operational/thermal cycles. A drawback of increased clamping force is that it tends to deform or warp the disks.
One other attempt to reduce slippage has been to oversize the central opening of the disks and space the disks away from the sidewall of the spindle hub, so that as the disks expand and contract radially, they do not come into contact with the outerwall surface of the hub.
None of these prior approaches has worked completely satisfactorily to solve the problem of disk slippage and movement incident to thermal cycling of the disk drive.